Electrical submersible pumps are commonly used in hydrocarbon producing wells. A typical pump assembly includes an electrical motor having a rotating drive shaft that drives the pump. The pump is often a centrifugal pump having a large number of stages. Each stage has a nonrotating diffuser and a rotating impeller. The motor has a drive shaft that couples to the pump shaft to rotate the impellers. The motor may have lengths up to 30 feet or more. Radial motor bearings support the motor shaft along the lengths. A dielectric fluid in the motor lubricates the motor bearings. A pressure equalizer mounts to the motor to reduce a pressure difference between the dielectric lubricant in the motor and the well fluid on the exterior. A shaft seal, usually at an end of the pressure equalizer, seals around the drive shaft to prevent the entry of well fluids into the motor lubricant.
Another type of pump assembly comprises a progressive cavity pump, which has a helical rotor that rotates within a double helical passage of an elastomeric member, also called a stator. An electrical motor may be coupled to the rotor via a gear box and flex shaft, which accommodates orbital motion of the rotor.
In one type of installation, the assembled pump and motor are attached to a lower end of a string of production tubing and lowered into casing of the well. A power cable extends alongside the production tubing to the motor to supply power. If repair or replacement to the pump is required, normally a workover rig is required to pull the tubing and the pump and motor assembly.
In another type of installation, the motor is secured to the lower end of production tubing. The pump may be lowered and retrieved through the production tubing. The pump has an engaging member on its lower end that engages the upper end of the drive shaft of the motor.